Getting a `sphinx.errors.ExtensionError` for `sphinx>=9`

[namurphy]

While attempting to upgrade PlasmaPy documentation builds to sphinx>=9 in PlasmaPy/PlasmaPy#3072, we ended up starting to get the following error:

      File "/home/runner/work/PlasmaPy/PlasmaPy/.nox/docs_upstream-sphinx/lib/python3.13/site-packages/sphinx/events.py", line 423, in emit
        raise ExtensionError(
        ...<4 lines>...
        ) from exc
    sphinx.errors.ExtensionError: Handler <plasmapy_sphinx.automodsumm.generate.GenDocsFromAutomodsumm object at 0x7f5c496cb8c0> for event 'builder-inited' threw an exception (exception: generate_autosummary_content() got an unexpected keyword argument 'registry')

I looked at Sphinx's incompatible changes for v9.0.0 and tried setting autodoc_use_legacy_class_based = True, but the problem didn't seem to go away.

I tried building the docs for bapsflib and ran into the same issue.

@rocco8773 — would you be able to look into this? Many thanks!